Folate and its synthetic form, folic acid, is a water-soluble B vitamin which is involved in DNA synthesis and repair, and in regulation of gene expression through DNA methylation as a methyl donor. Despite of confirmed beneficial effect of folate/folic acid supplementation on the prevention of neural tube defect, concerns have been raised recently that folic acid food fortification and high prevalence of folic acid intake with multivitamins and supplements may lead to excessively high intakes, which may promote carcinogenesis or cancer progression. On May, 2015, the NIH National Toxicology Program (NTP) and Office of Dietary Supplements (ODS) convened an expert panel to identify research needs related to the safe use of high intakes of folic acid based on consideration of the state of the science. The Cancer Subpanel concluded, ?There is consistent enough suggestion in human studies of an adverse effect on cancer growth from supplemental folic acid to justify further research? (NTP Monograph: Identify Research Needs for Assessing Safe Use of High Intakes of Folic Acid. Systematic reviews and meta-analyses suggested a positive association between serum folate levels and risk of prostate cancer. Circulating unmetabolized folic acid (cUMFA) is a by-product of high intake of folic acid, and has been associated with reduction of cytotoxicity of natural killer cells, which may be a mechanism by which high levels of folic acid promote carcinogenesis. Study has shown that participants with dihydrofolate reductase (DHFR) 19bpdel/del were at increased risk of having elevated levels of cUMFA, particularly when daily folic acid intake exceeded 500 micrograms. African Americans (AAs) are diagnosed with aggressive prostate cancer more often and have more than twice the prostate cancer mortality rates as European Americans (EAs). Two published studies demonstrated that DHFR 19bpdel/del frequency is more prevalent among AAs than EAs, which may partially explain the elevated prostate cancer rates among AAs. Therefore, the hypothesis of this application is that excess folic acid intake, as measured by both diet/supplement intake and plasma cUMFA, will be associated with increased odds of high aggressive prostate cancer, and that this effect will be modified by the DHFR polymorphism. This study capitalizes on a large, previously- conducted, well-characterized, case-only study of approximately equal number of AAs and EAs to examine folate/folic acid intake, biomarkers, genetic polymorphisms and racial differences in prostate cancer aggressiveness. The specific aims are to 1) examine the association between folate/folic acid intake from food and supplements in the year prior to diagnosis and prostate cancer aggressiveness in EAs and AAs, 2) evaluate whether there is significant difference in the prevalence of DHFR 19bpdel/del polymorphism between AA and EA; and whether levels of cUMFA are predicted by total intake of folic acid modulated by the DHFR polymorphism, and 3) examine the association between plasma folate/cUMFA and prostate cancer aggressiveness in EAs and AAs. The project represents an innovative approach to identify modifiable risk factors and genetic markers to address an important public health issue, and is strengthened by the collaboration of a strong, multidisciplinary team of investigators. Finding of this study, if hypothesis confirmed, will shed light into concerns about the safety of folic acid supplement and to inform dietary guidelines for cancer prevention.